This invention relates in general to radio receivers, and in particular to narrow band zero intermediate frequency (ZIF) receivers. Specifically, certain embodiments of this invention relate to narrow band ZIF receivers that employ automatic gain control (AGC).
Narrow band ZIF receivers tend to have slow AGC loop settling times. This is due in part to the closed loop nature of AGC systems and the fact that narrow bandwidth filters tend to add lengthy phase delays near the filter corner frequencies, thereby placing stability limitations on the maximum loop bandwidth of an AGC loop. For narrow band ZIF receivers, AGC settling times in the 4-6 millisecond range are considered exceptionally short. Notwithstanding, there are several emerging applications where settling times under 2 milliseconds are required. AGC Loops wide enough to theoretically achieve the desired settling times are typically unstable or underdamped and therefore induce excessive ripple in the AGC loop gain control signal. An alternative solution suggests switching to a faster AGC loop during settling and then switching back to the slower loop during normal operation. Unfortunately, when a fast AGC loop is switched to a slower AGC loop, ripple in the gain control signal due to fast loop tracking of the amplitude modulated (AM) signal can leave the control signal at an arbitrarily high level. Additional settling time is required to recover from this overshoot condition. As will be appreciated by those of ordinary skill in the art, this additional settling time only operates to defeat the purpose of adapting the fast loop in the first instance. For these reasons, it would be extremely advantageous to provide an apparatus for acquiring a rapid AGC response (settling time) in a narrow band ZIF receiver.